2,507 research outputs found
Deep dielectric charging of regolith within the Moon\u27s permanently shadowed regions
Abstract Energetic charged particles, such as galactic cosmic rays (GCRs) and solar energetic particles (SEPs), can penetrate deep within the lunar surface, resulting in deep dielectric charging. This charging process depends on the GCR and SEP currents, as well as on the regolith\u27s electrical conductivity and permittivity. In permanently shadowed regions (PSRs) near the lunar poles, the discharging timescales are on the order of a lunation (âŒ20 days). We present the first predictions for deep dielectric charging of lunar regolith. To estimate the resulting subsurface electric fields, we develop a data-driven, one-dimensional, time-dependent model. For model inputs, we use GCR data from the Cosmic Ray Telescope for the Effects of Radiation on board the Lunar Reconnaissance Orbiter and SEP data from the Electron, Proton, and Alpha Monitor on the Advanced Composition Explorer. We find that during the recent solar minimum, GCRs create persistent electric fields up to âŒ700 V/m. We also find that large SEP events create transient but strong electric fields (â„106 V/m) that may induce dielectric breakdown. Such breakdown would likely result in significant modifications to the physical and chemical properties of the lunar regolith within PSRs. Key Points Energetic charged particles deep dielectrically charge the lunar regolithWe model the resulting subsurface electric fieldsThe electric fields may be great enough to induce dielectric breakdown
Thermal modeling of subduction zones with prescribed and evolving 2D and 3D slab geometries
The determination of the temperature in and above the slab in subduction
zones, using models where the top of the slab is precisely known, is important
to test hypotheses regarding the causes of arc volcanism and intermediate-depth
seismicity. While 2D and 3D models can predict the thermal structure with high
precision for fixed slab geometries, a number of regions are characterized by
relatively large geometrical changes. Examples include the flat slab segments
in South America that evolved from more steeply dipping geometries to the
present day flat slab geometry. We devise, implement, and test a numerical
approach to model the thermal evolution of a subduction zone with prescribed
changes in slab geometry over time. Our numerical model approximates the
subduction zone geometry by employing time dependent deformation of a B\'ezier
spline which is used as the slab interface in a finite element discretization
of the Stokes and heat equations. We implement the numerical model using the
FEniCS open source finite element suite and describe the means by which we
compute approximations of the subduction zone velocity, temperature, and
pressure fields. We compute and compare the 3D time evolving numerical model
with its 2D analogy at cross-sections for slabs that evolve to the present-day
structure of a flat segment of the subducting Nazca plate
Essays on Industrial Organization: Regulatory Uncertainty and Dynamic Decision-Making by Firms.
This dissertation is composed of three papers concerned with how firms respond to uncertainty and dynamic interactions. These are areas that have long been important research topics among industrial economists. The justification for this is obvious: the course of events is rarely deterministic, and the past often exerts a marked influence on present decisions. Both factors lead actors to adopt very different strategies from what they would do in a deterministic, one-shot world.
In my first paper, I show how varying levels of uncertainty about the regulatory environment shift the boundaries of the firm by affecting the incentives for both ownership and control. In my second paper, I use a theoretical model to explore how uncertainty about the timing of a regulatory change affects competition between oligopolists. I find that for this form of uncertainty, the standard âreal optionsâ prediction of lower investment in uncertain environments does not necessarily hold. In my third paper, I examine the theory of entry deterrence by means of spatial preemption. I first develop a theoretical model of entry deterrence among multiproduct oligopolists that incorporates the stylized facts of consumer heterogeneity and brand awareness. The desirability of different strategies will vary depending on the intensity of consumersâ brand preferences. I test the modelâs predictions using data on retail hotel markets, where properties are differentiated by their quality and related through the ownership of their brands. My findings strongly support the modelâs predictions about when spatial preemption will be a credible strategy.Ph.D.Business AdministrationUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/77848/1/nathanew_1.pd
Postmortem tissue distribution of morphine and its metabolites in a series of heroin related deaths
The abuse of heroin (diamorphine) and heroin deaths are growing around the world. The interpretation of the toxicological results from suspected heroin deaths is notoriously difficult especially in cases where there may be limited samples. In order to help forensic practitioners with heroin interpretation we determined the concentration of morphine (M), morphineâ3âglucuronide (M3G) and morphineâ6âglucuronide (M6G) in blood (femoral and cardiac), brain (thalamus), liver (deep right lobe), bone marrow (sternum), skeletal muscle (psoas) and vitreous humor in 44 heroin related deaths. The presence of 6âmonoacetylmorphine (6âMAM) in any of the postmortem samples was used as confirmation of heroin use. Quantitation was carried out using a validated LCâMS/MS method with solid phase extraction. We also determined the presence of papaverine, noscapine and codeine in the samples, substances often found in illicit heroin and that may help determine illicit heroin use. The results of this study show that vitreous is the best sample to detect 6âMAM (100% of cases), and thus heroin use. The results of the M, M3G and M6G quantitation in this study allow a degree of interpretation when samples are limited. However in some cases it may not be possible to determine heroin/morphine use as in 4 cases in muscle (3 cases in bone marrow) no morphine, morphineâ3âglucuronide or morphineâ6âglucuronide was detected, even though they were detected in other case samples. As always postmortem cases of suspected morphine/heroin intoxication should be interpreted with care and with as much case knowledge as possible
A divergence free -RIPG stream function formulation of the incompressible Stokes system with variable viscosity
Pointwise divergence free velocity field approximations of the Stokes system
are gaining popularity due to their necessity in precise modelling of physical
flow phenomena. Several methods have been designed to satisfy this requirement;
however, these typically come at a greater cost when compared with standard
conforming methods, for example, because of the complex implementation and
development of specialized finite element bases. Motivated by the desire to
mitigate these issues for 2D simulations, we present a -interior penalty
Galerkin (IPG) discretization of the Stokes system in the stream function
formulation. In order to preserve a spatially varying viscosity this approach
does not yield the standard and well known biharmonic problem. We further
employ the so-called robust interior penalty Galerkin (RIPG) method; stability
and convergence analysis of the proposed scheme is undertaken. The former,
which involves deriving a bound on the interior penalty parameter is
particularly useful to address the growth in the
condition number of the discretized operator. Numerical experiments confirming
the optimal convergence of the proposed method are undertaken. Comparisons with
thermally driven buoyancy mantle convection model benchmarks are presented
The first cosmic ray albedo proton map of the Moon
[1] Neutrons emitted from the Moon are produced by the impact of galactic cosmic rays (GCRs) within the regolith. GCRs are high-energy particles capable of smashing atomic nuclei in the lunar regolith and producing a shower of energetic protons, neutrons and other subatomic particles. Secondary particles that are ejected out of the regolith become âalbedoâ particles. The neutron albedo has been used to study the hydrogen content of the lunar regolith, which motivates our study of albedo protons. In principle, the albedo protons should vary as a function of the input GCR source and possibly as a result of surface composition and properties. During the LRO mission, the total detection rate of albedo protons between 60 MeV and 150 MeV has been declining since 2009 in parallel with the decline in the galactic cosmic ray flux, which validates the concept of an albedo proton source. On the other hand, the average yield of albedo protons has been increasing as the galactic cosmic ray spectrum has been hardening, consistent with a disproportionately stronger modulation of lower energy GCRs as solar activity increases. We construct the first map of the normalized albedo proton emission rate from the lunar surface to look for any albedo variation that correlates with surface features. The map is consistent with a spatially uniform albedo proton yield to within statistical uncertainties
Radiation modeling in the Earth and Mars atmospheres using LRO/CRaTER with the EMMREM Module
Abstract We expand upon the efforts of Joyce et al. (2013), who computed the modulation potential at the Moon using measurements from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument on the Lunar Reconnaissance Orbiter (LRO) spacecraft along with data products from the Earth-Moon-Mars Radiation Environment Module (EMMREM). Using the computed modulation potential, we calculate galactic cosmic ray (GCR) dose and dose equivalent rates in the Earth and Mars atmospheres for various altitudes over the course of the LRO mission. While we cannot validate these predictions by directly comparable measurement, we find that our results conform to expectations and are in good agreement with the nearest available measurements and therefore may be used as reasonable estimates for use in efforts in risk assessment in the planning of future space missions as well as in the study of GCRs. PREDICCS (Predictions of radiation from REleASE, EMMREM, and Data Incorporating the CRaTER, COSTEP, and other solar energetic particles measurements) is an online system designed to provide the scientific community with a comprehensive resource on the radiation environments of the inner heliosphere. The data products shown here will be incorporated into PREDICCS in order to further this effort and daily updates will be made available on the PREDICCS website (http://prediccs.sr.unh.edu). Key Points We model GCR dose and dose equivalent rates in Earth and Mars atmospheres Dose rates are in reasonable agreement with nearby measurements Data products will soon be made available on PREDICCS website
Measurements of galactic cosmic ray shielding with the CRaTER instrument
[1] The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument aboard the Lunar Reconnaissance Orbiter has been measuring energetic charged particles from the galactic cosmic rays (GCRs) and solar particle events in lunar orbit since 2009. CRaTER includes three pairs of silicon detectors, separated by pieces of tissue-equivalent plastic that shield two of the three pairs from particles incident at the zenith-facing end of the telescope. Heavy-ion beams studied in previous ground-based work have been shown to be reasonable proxies for the GCRs when their energies are sufficiently high. That work, which included GCR simulations, led to predictions for the amount of dose reduction that would be observed by CRaTER. Those predictions are compared to flight data obtained by CRaTER in 2010â2011
Dielectric breakdown weathering of the Moon\u27s polar regolith
Abstract
Galactic cosmic rays and solar energetic particles (SEPs) can charge the Moon\u27s subsurface, a process expected to be particularly important in the polar regions. Experiments have shown that sufficient fluences (i.e., time-integrated fluxes) of energetic charged particles can cause dielectric breakdown, in which the electric field rapidly vaporizes small, filamentary channels within a dielectric. Lunar regolith has both the characteristics and, in some polar locations, the environment needed to make breakdown likely. We combine the Jet Propulsion Laboratory proton fluence model with temperature measurements from the Lunar Reconnaissance Orbiter\u27s (LRO\u27s) Diviner instrument and related temperature modeling to estimate how often breakdown occurs in the polar regions. We find that all gardened regolith within permanently shadowed regions (PSRs) has likely experienced up to 2Ă106 SEP events capable of causing breakdown, while the warmest polar regions have experienced about 2 orders of magnitude fewer events. We also use measurements from the Cosmic Ray Telescope for the Effects of Radiation on LRO to show that at least two breakdown-inducing events may have occurred since LRO arrived at the Moon in 2009. Finally, we discuss how such âbreakdown weatheringâ may increase the percentage of fine and monomineralic grains within PSRs; explain the presence of so-called âfairy castleâ regolith structures; and contribute to other low-albedo features detected by LRO\u27s Lyman Alpha Mapping Project, possibly establishing a correlation between these features and the average temperatures within craters that are only partly in permanent shadow
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